Method for producing a re-shaped component from a manganese-containing flat steel product and such a component

ABSTRACT

The invention relates to a method for producing a component from a medium manganese flat steel product having 4 to less than 10 wt. % Mn, 0.0005 to 0.9 wt. % C, 0.02 to 10 wt. % Al, the remainder iron, including unavoidable steel-accompanying elements, and having a TRIP effect at room temperature. In order to produce a component, which is distinguished by very high strengths and an increased residual strain and re-shaping capacity, the flat steel product, according to the invention, is re-shaped by at least one re-shaping step to form a component and, before and/or during and/or after the at least one re-shaping step, the flat steel product is cooled down to a temperature of the flat steel product of less than room temperature to −196° C. The invention further relates to a component produced by this method and to a use for said components.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2017/070777, filed Aug. 16, 2017, which designated the UnitedStates and has been published as International Publication No. WO2018/050387 and which claims the priority of German Patent Application,Serial No. 10 2016 117 507.2, filed Sep. 16, 2016, pursuant to 35 U.S.C.119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a method for producing a component consistingof a medium manganese flat steel product comprising 4 to less than 10wt. % Mn, 0.0005 to 0.9 wt. % C, 0.02 to 10 wt. % Al, with the remainderbeing iron including unavoidable steel-associated elements and with aTRIP effect at room temperature. The invention also relates to acomponent produced by this method and a use for these components.

European patent application EP 2 383 353 A2 discloses a flat steelproduct consisting of a manganese steel which consists of the followingelements (contents in weight percent in relation to the steel melt): C:to 0.5; Mn: 4 to 12.,0; Si: up to 1.0; Al: up to 3.0; Cr: 0.1 to 4.0;Cu: up to 4.0; Ni: up to 2.0; N: up to 0.05; P: up to 0.05; S: up to0.01, with the remainder being iron and unavoidable impurities.Optionally, one or more elements from the group “V, Nb, Ti” areprovided, wherein the sum of the contents of these elements is at mostequal to 0.5. This steel is said to be characterised in that it can beproduced in a more cost-effective manner than high manganese steels andat the same time has high elongation at fracture values and, associatedtherewith, a considerably improved deformability. After quenching of theflat steel product in oil or water at room temperature, the flat steelproduct has a tensile strength of 900 to 1500 MPa at an elongation atfracture A80 of at least 4%.

German laid-open document DE 10 2012 111 959 A1 describes a method forproducing a motor vehicle component. For this purpose, a steel havingTWIP properties at room temperature, a high manganese content of 10 to30 wt. %, a maximum aluminium content of 8 wt. % and a maximum carboncontent of 2 wt. % is preferably used. In the method, a steel sheetplate is tempered at least partially to a cold forming temperature of+30° C. to −250° C. Subsequently, the steel sheet plate is formed intothe motor vehicle component at the cold forming temperature, wherebymartensite formation is at least partially induced.

Also, German laid-open document DE 10 2012 013 113 A1 already describesso-called TRIP steels which have a predominantly ferritic basicmicrostructure having incorporated residual austenite which can convertinto martensite during deformation (TRIP effect). Owing to its intensecold-hardening, the TRIP steel achieves high values for uniformelongation and tensile strength. TRIP steels are suitable for use interalia in structural components, chassis components and crash-relevantcomponents of vehicles, as sheet metal blanks and as tailored weldedblanks.

Furthermore, German patent DE 10 2013 104 298 B4 describes rollprofiling, also referred to as roll forming, as a method for deformingmedium or high manganese steel strips. Roll forming or roll profiling isa continuous bending method, in which the steel strips are deformed intoa desired final shape by a multiplicity of roller pairs step-by-step upto roll profile products. Frequently, a combination of the roll formingwith other production methods, such as e.g. punching, longitudinalwelding or embossing was used in order to produce virtually any profileshapes even with cross-sections which vary over the component length. Inorder to achieve a considerable increase in strength by means of rollprofiling, at least sections of the steel strip are to be cooled, priorto and/or during the roll profiling, to a maximum temperature of −20° C.or less, e.g. to a temperature of −40° C. to −180° C.

A further known deformation process, the so-called internalhigh-pressure forming, is described in laid-open document DE 10 2008 014213 A1 e.g. with the aid of the internal high-pressure forming of pipes.Tubular workpieces are hereby placed into at least two-part tools andare subjected on the inner side to an active medium under a highpressure. The workpiece is hereby expanded, formed into an engraving orgeometry of the tool, partially pushed further and thus acquires theshape of the tool. The material must be configured such that a highdeformation can also be absorbed locally without material failure.

Proceeding therefrom, the object of the present invention is to providea method for producing a component consisting of a medium manganese flatsteel product, a component produced by this method and a use therefor,which component is characterised by very high strengths with anincreased residual elongation and deformation capability.

SUMMARY OF THE INVENTION

In accordance with the invention, the object is achieved by a method forproducing a component of a manganese-containing flat steel productcomprising 4 to less than 10 wt. % Mn, 0.0005 to 0.9 wt. % C, 0.02 to 10wt. % Al, with the remainder being iron including unavoidablesteel-associated elements and with a TRIP effect at room temperature,the method comprising the steps of:

-   -   deforming the flat steel product to form a component by means of        a first deforming step    -   cooling the flat steel product prior to and/or during and/or        after the at least one deforming step to a temperature of the        flat steel product of less than room temperature to −196° C.

A method according to the invention ensures that the component has veryhigh strengths with an increased residual elongation and deformationcapability. By lowering the temperature prior to, during or after apreferably final deformation, the TRIP effect is preferred in terms ofenergy and a larger proportion of the austenite in the medium manganesesteel converts into martensite. This deformation in the low temperaturerange in accordance with the invention is associated with the fact thatat the same time the deformation capability and the strength areincreased by the TRIP effect. In general, the component is alsosignificantly hardened. The strength is increased during cooling on itsown without deformation by means of a temperature-induced conversion ofmetastable austenite into martensite. The austenite proportion, aftersupercooling and optional deformation have been performed, is at least10% lower, in relation to the initial austenite content, than theaustenite content at room temperature or a comparable deformation atroom temperature. Athermal martensite formation for increasing thestrength whilst at the same time decreasing the elongation/toughness isalso achieved by means of cooling after deformation.

Advantageous embodiments of the invention are described in the dependentclaims.

On the whole, the deformation at the temperatures which are low inaccordance with the invention ensures that the TRIP effect is used in atargeted manner in order, at low temperatures below RT, to effectsignificant hardening and to obtain a higher martensite proportion inthe microstructure.

In an advantageous manner, provision is made that the flat steel productis deformed to form a component in at least one deforming step at atemperature of the flat steel product of less than room temperature to−196° C., preferably less than 0 to −196° C. For this purpose, the flatsteel product is cooled, prior to at least one deforming step, to atemperature of less than room temperature to −196° C., preferably lessthan 0° C. to −196° C. Thereafter, cooling of the flat steel productprior to and/or during deformation is preferred. In a particularlypreferred manner, by reason of the increase in strength which occurs thedeformation at the inventive low temperatures is performed only in oneor a plurality of final deforming steps because the deformationcapability is reduced by the increase in strength.

In addition and in combination, provision can be made that the flatsteel product is deformed to form a component by means of a first orfurther deforming steps at a temperature of the flat steel product of60° C. to below Ac3, preferably 60° C. to 450° C. In this case, the flatsteel product is pre-heated, prior to the first or further deformingstep, to a temperature of 60° C. to below Ac3, preferably to 60° C. to450° C.

A reduction in the required deformation forces is also associated withthe increase in the temperature prior to the first deformation. Anincrease in the residual deformation capability of the deformedcomponents with tensile strengths of greater than 800 MPa to 2000 MPa atelongations of fracture of greater than 3% is also produced in theregions which are deformed to the greatest extent. The flat steelproduct can be pre-heated for the coil or the wound strip or panelmaterial. By way of the deformation with pre-heating, in accordance withthe invention, of the flat steel product prior to the first deformationstep, conversion of the metastable austenite into martensite (TRIPeffect) is completely or partially suppressed during the deformationprocess, wherein deformation twins (TWIP effect) can form in theaustenite. The inventive and advantageous reduction in the deformationforces is hereby achieved, and the overall deformation capability isincreased.

In a further variant of the method, provision is made that the flatsteel product is deformed to form a component by means of furtherdeforming steps at a temperature of the flat steel product of roomtemperature to below Ac3, preferably room temperature to 450° C. As aresult, deformation twins can be introduced in a targeted manner and atroom temperature continue to convert into martensite and permit a higherdegree of deformation, whereby the energy absorption capability ofcomponents is increased.

In conjunction with the present invention, room temperature is definedas lying in the range between 15 to 25° C.

In a particularly advantageous manner, provision is made that the flatsteel product is deformed to form a component by means of the furtherindividual deforming steps at different temperatures which in each casecan also be locally defined. A targeted and local adjustment of strengthand elongation properties of the component can hereby optionally beachieved by varying the deformation temperature. Therefore, propertiesare adjusted locally in a targeted manner by cooling or heating.Primarily higher strengths are achieved by means of targeted cooling andhigher residual elongations and higher deformation capabilities areachieved by means of local heating.

By virtue of the fact that the cooling is performed only partially, anincrease in the strength characteristic values characterised by theyield strength/elasticity limit and/or tensile strength is achieved inspecific regions of the component. This corresponds to so-calledtailored cooling.

In one variant, provision is made that the flat steel product is cooledand/or pre-heated on one side. As a result, a hard layer can be producedon a ductile component. Alternatively, provision can be made that theflat steel product is pre-heated and/or cooled on both sides.Preferably, provision is made that the flat steel product is cooled onboth sides such that a temperature gradient is present over thecross-section of the flat steel product.

In order to maintain the temperature window, in accordance with theinvention, for the deformation, the flat steel product can beintermediately heated or intermediately cooed during deformation betweenthe deformation steps to temperatures between −196° C. to belowAc3—depending upon the desired procedure,

In a particularly advantageous manner, the method is suitable to deformthe flat steel product by means of roll deforming,

During the roll deforming, the flat steel product undergoes at least oneof creasing, compression, thickness reduction in regions, embossing,punching or channelling or combinations thereof in a multiplicity ofsuccessive deformation or processing steps. Components in the form ofdosed profiles can also be produced which for this purpose areoptionally welded, preferably longitudinal seam welded, after the rolldeforming.

The method is also suitable for deforming tubes, which are produced fromthe flat steel product, by means of internal high-pressure forming whichpreferably occurs by means of solid, liquid or gaseous active media. Ina known manner, the flat steel product, in particular a rolled hot stripor cold strip, is formed for this purpose into a slit tube and thenlongitudinal seam welded to form a tube or alternatively formed into aspiral and spiral seam welded to form a tube. Preferably but optionally,the tube is then annealed (500 to 850° C., 30 seconds to 12 h)immediately after the longitudinal seam welding or spiral seam welding,e.g. inductively or in the continuous furnace or in a stationary furnaceunit, such as e.g. in the hearth furnace or muffle furnace.Alternatively, with a low degree of deformation and sufficient remainingresidual ductility for the subsequent internal high-pressure forming,the annealing step can be omitted and thus further processing in thehardened state can be performed. Then, internal high-pressure forming inaccordance with the invention is performed at a preferred temperature of60 to 450° C. Heating is preferably performed via the active medium.Deforming can be preformed in a plurality of steps. After warm internalhigh-pressure forming, the component preferably still has at least 50%of the initial austenite content. An advantageous temperature range forthe internal high-pressure forming is between 60 and 450° C.

In relation to the component obtained via roll deforming or internalhigh-pressure forming, the following dependencies upon tensile strengthRm in MPa and elongation at fracture A80 in % are produced:

Rm of 700 to 800 MPa: Rm×A80≥15400 up to 50000 MPa %

Rm of over 800 to 900 MPa: Rm×A80≥14400 up to 50000 MPa %

Rm of over 900 to 1100 MPa: Rm×A80≥13500 up to 45000 MPa %

Rm of over 1100 to 1200 MPa: Rm×A80≥13200 up to 45000 MPa %

Rm of over 1200 to 1350 MPa: Rm×80≥11200 up to 45000 MPa %

Rm of over 1350 to 1800 MPa: Rm×A80≥8000 up to 45000 MPa %

Rm of over 1800 MPa: Rm×A80≥7200 up to 30000 MPa %

In a particularly preferred manner, provision is made that the flatsteel product is produced with the following chemical composition (inwt. %) in order to achieve in particular the described advantages:

C: 0.0005 to 0.9, preferably 0.05 to 0.35

Mn: 4 to less than 10, preferably greater than 5 to less than 10

Al: 0.02 to 10, preferably 0.05 to 5, particularly preferred greaterthan 0.5 to 3

with the remainder being iron including unavoidable steel-associatedelements,

with optional addition by alloying of:

Si: 0 to 6, preferably 0.05 to 3, in a particularly preferred manner 0.1to 1.5

Cr: 0 to 6, preferably 0.1 to 4, particularly preferred greater than 0.5to 2.5

Nb: 0 to 1, preferably 0.005 to 0.4, particularly preferred 0.01 to 0.1

V: 0 to 1.5, preferably 0.005 to 0.6, particularly preferred 0.01 to 0.3

Ti: 0 to 1.5, preferably 0.005 to 0.6, particularly preferred 0.01 to0.3

Mo: 0 to 3, preferably 0.005 to 1.5, particularly preferred 0.01 to 0.6

Sn: 0 to 0.5, preferably less than 0.2, particularly preferred less than0.05

Cu: 0 to 3, preferably less than 0.5, particularly preferred less than0.1

W: 0 to 5, preferably 0.01 to 3, particularly preferred 0.2 to 1.5

Co: 0 to 8, preferably 0.01 to 5, particularly preferred 0.3 to 2

Zr: 0 to 0.5, preferably 0.005 to 0.3, particularly preferred 0.01 to0.2

Ta: 0 to 0.5, preferably 0.005 to 0.3, particularly preferred 0.01 to0.1

Te: 0 to 0.5, preferably 0.005 to 0.3, particularly preferred 0.01 to0.1

B: 0 to 0.15, preferably 0.001 to 0.08, particularly preferred manner0.002 to 0.01

P: less than 0.1, preferably less than 0.04

S: less than 0.1, preferably less than 0.02

N: less than 0.1, preferably less than 0.05.

This flat steel product consisting of the medium manganese TRIP(TRansformation Induced Plasticity) and/or TWIP (TWinning InducedPlasticity) steel has excellent cold-formability and warm-formability,increased resistance to hydrogen-induced delayed crack formation(delayed fracture), to hydrogen embrittlement after deformation and toliquid metal embrittlement (LME) during welding,

In a conventional manner, the previously described flat steel product isproduced by a production route described hereinafter:

-   -   melting and alloying a steel melt with the above-described        chemical composition in a, via the process route, blast furnace        steel plant or electric arc furnace steel plant with optional        vacuum treatment of the melt;    -   casting the steel melt to form a pre-strip by means of a        horizontal or vertical strip casting process approximating the        final dimensions or casting the steel melt to form a slab or        thin slab by means of a horizontal or vertical slab or thin slab        casting process,    -   heating the pre-strip to a rolling temperature of 1050 to        1250° C. or in-line rolling out of the casting heat (first        heat),    -   hot rolling the pre-strip or the slab or the thin slab to form a        hot strip having a thickness of 20 to 0.8 mm at a final rolling        temperature of 1050 to 800° C., reeling the hot strip at a        temperature of more than 100 to 800° C.,    -   acid-cleaning the hot strip,    -   annealing the hot strip in a continuous annealing installation        or batch-type—or discontinuous—annealing installation for an        annealing time of 1 min to 24 h and at temperatures of 500 to        840° C.,    -   optionally cold rolling the hot strip at room temperature,        preferably with pre-heating to 60 to below Ac3 temperature,        preferably 60 to 450° C. prior to the first rolling pass to        reduce the rolling forces and form deformation twins in the        austenite and, as required, cooling or heating between the        rolling passes to 60° C. to below the Ac3 temperature,        preferably 60 to 450° C.,    -   optionally annealing at 500 to 840° C. for 1 min to 24 h in a        continuous annealing installation or batch-type—or        discontinuous—annealing installation,    -   optionally electrolytically galvanising or hot-dip galvanising        the steel strip or applying another organic or inorganic        coating.

Then, the flat steel product is deformed in accordance with theinvention into a component.

The flat steel product produced by this production route has amicrostructure with an austenite content of 10 to 80%, 20 to 90%martensite, ferrite and bainite, wherein at least 30% of the martensiteis present as annealed martensite. Preferably, the microstructure has 40to 80% austenite, less than 20% ferrite/bainite, with the rest beingmartensite.

Typical thickness ranges for the pre-strip are 1 mm to 35 mm and forslabs and thin slabs they are 35 mm to 450 mm. Provision is preferablymade that the slab or thin slab is hot rolled to form a hot strip havinga thickness of 20 mm to 0.8 mm or the pre-strip, cast to approximatelythe final dimensions, is hot rolled to form a hot strip having athickness of 8 mm to 0.8 mm. The cold strip has a thickness of typicallyless than 3 mm, preferably 0.1 to 1.4 mm.

In the context of the above method in accordance with the invention, apre-strip produced with the two-roller casting process and approximatingthe final dimensions and having a thickness of less than or equal to 3mm, preferably 1 mm to 3 mm is already understood to be a hot strip. Thepre-strip thus produced as a hot strip does not have a cast structureowing to the introduced deformation of the two rollers running inopposite directions. Hot rolling thus already takes place in-line duringthe two-roller casting process which means that separate heating and hotrolling is not necessary.

The cold rolling of the hot strip can take place at room temperature oradvantageously at elevated temperature with one heating process prior tothe first rolling pass and/or with heating processes in a subsequentrolling pass or between several rolling passes. The cold rolling atelevated temperature is advantageous in order to reduce the rollingforces and to aid the formation of deformation twins (TWIP effect).Advantageous temperatures of the material being rolled prior to thefirst rolling pass are 60° C. to below Ac3 temperature, preferably 60 to450° C.

If the cold rolling is performed in a plurality of rolling passes, it isadvantageous to intermediately heat or cool the steel strip between therolling passes to a temperature of 60° C. to below Ac3 temperature,preferably 60° C. to 450° C. because the TWIP effect is brought to bearin a particularly advantageous manner in this range. Depending upon therolling speed and degree of deformation, intermediate heating, e.g. atvery low degrees of deformation and rolling speeds, and also additionalcooling, caused by heating the material with rapid rolling and highdegrees of deformation, can be performed.

After cold rolling of the hot strip at room temperature, the steel stripis to be annealed in a continuous annealing installation orbatch-type—or other discontinuous—annealing installation advantageouslyfor an annealing time of 1 min to 24 h, preferably less than 10 min, andat temperatures of 500 to 840° C., in order to restore sufficientforming properties. If required in order to achieve specific materialproperties, this annealing procedure can also be performed with thesteel strip rolled at elevated temperature.

After the annealing treatment, the steel strip is advantageously cooledto a temperature of 250° C. to room temperature and subsequently, ifrequired, in order to adjust the required mechanical properties, in thecourse of ageing treatment, is reheated to a temperature of 300 to 450°C., is maintained at this temperature for up to 5 min and subsequentlyis cooled to room temperature. The ageing treatment can be performedadvantageously in a continuous annealing installation.

The flat steel product produced in this manner can optionally beelectrolytically galvanised or hot-dip galvanised. In one advantageousdevelopment, the steel strip produced in this manner acquires a coatingon an organic or inorganic basis instead of or after the electrolyticgalvanising or hot-dip galvanising. They can be e.g. organic coatings,synthetic material coatings or lacquers or other inorganic coatings,such as e.g. iron oxide layers.

In accordance with the invention, a deformed component can be producedby the above-described method. The component which is preferablydeformed at elevated temperature has, with the same degree ofdeformation, at least the same or higher strength properties (yieldstrength/elasticity limit and/or tensile strength) as/than a componentdeformed at room temperature, wherein the elongation at fracture is atleast 10% higher in comparison with the deformation at room temperature.In a similar manner, it is possible to set comparable characteristicvalues for the elongation at fracture, wherein the characteristic valuefor the strength (yield strength/elasticity limit and/or tensilestrength) is, in comparison, 10% above the characteristic values ofdeformation at room temperature. The warm-formed component has anincreased resistance to hydrogen-induced embrittlement and delayed crackformation because the TRIP effect is at least partially suppressed.Also, liquid metal embrittlement does not occur during welding.

The invention renders it possible to produce a high-strength componentwhich has a residual elongation and/or residual toughness considerablyimproved in comparison with low-alloyed steels of the same strengthclass and is considerably more cost-effective in comparison with highmanganese steels which have alloy contents >=10 wt. % Mn and/orhigh-Cr-alloyed and/or Cr-Ni or other high-alloyed steels currently usedfor such applications,

In accordance with the invention, a use of a component produced by thepreviously described method is advantageously provided in the automotiveindustry, rail vehicle construction, shipbuilding, plant design,infrastructure, the aerospace industry, household appliances and intailored welded blanks.

A steel strip produced according to the method in accordance with theinvention advantageously has an elasticity limit Rp0.2 of 300 to 1350MPa, a tensile strength Rm of 1100 to 2200 MPa and an elongation atfracture A80 of more than 4 to 41%, wherein high strengths tend to beassociated with lower elongations at fracture and vice versa:

Rm of 700 to 800 MPa: Rm×A80≥15400 up to 50000 MPa %

Rm of over 800 to 900 MPa: Rm×A80≥14400 up to 50000 MPa %

Rm of over 900 to 1100 MPa: Rm×A80≥13500 up to 45000 MPa %

Rm of over 1100 to 1200 MPa: Rm×A80≥13200 up to 45000 MPa %

Rm of over 1200 to 1350 MPa: Rm×A80≥11200 up to 45000 MPa %

Rm of over 1350 to 1800 MPa: Rm×A80≥8000 up to 45000 MPa %

Rm of over 1800 MPa: Rm×A80≥7200 up to 30000 MPa %

The test piece type 2 having an initial measuring length of A80 was usedfor the elongation at fracture tests as per DIN 50 125.

The use of the term to in the definitions of the content ranges, such ase.g. 0.01 to 1 wt. %, means that the limit values—0.01 and 1 in theexample—are also included.

Alloy elements are generally added to the steel in order to influencespecific properties in a targeted manner. An alloy element can therebyinfluence different properties in different steels. The effect andinteraction generally depend greatly upon the quantity, presence offurther alloy elements and the solution state in the material. Thecorrelations are varied and complex. The effect of the alloy elements inthe alloy in accordance with the invention will be discussed in greaterdetail hereinafter. The positive effects of the alloy elements used inaccordance with the invention will be described hereinafter.

Carbon C: is required to form carbides, stabilises the austenite andincreases the strength. Higher contents of C impair the weldingproperties and result in the impairment of the elongation and toughnessproperties, for which reason a maximum content of 0.9 wt. %, preferably0.35 wt. %, is set. In order to achieve the desired combination ofstrength and elongation properties of the material, a minimum additionof 0.0005 wt. %, preferably 0.05 wt. %, is necessary.

Manganese Mn: stabilises the austenite, increases the strength and thetoughness and renders possible a deformation-induced martensiteformation and/or twinning in the alloy in accordance with the invention.Contents of less than 4 wt. % are not sufficient to stabilise theaustenite and thus impair the elongation properties, whereas withcontents of more than 10 wt. % and more the austenite is stabilised toomuch and as a result the strength properties, in particular the 0.2%elasticity limit, are reduced. For the manganese steel in accordancewith the invention having medium manganese contents, a range of 4 toless than 10 wt. %, preferably greater than 5 to 10, is preferred.

Aluminium Al: Al improves the strength and elongation properties,decreases the relative density and influences the conversion behaviourof the alloy in accordance with the invention. Excessively high contentsof Al impair the elongation properties. Higher Al contents alsoconsiderably impair the casting behaviour in the continuous castingprocess. This produces increased outlay when casting. High Al contentsdelay the precipitation of carbides in the alloy in accordance with theinvention. Therefore, an Al content of 0.02 to 10 wt. %, preferably 0.05to 5 wt. %, in a particularly preferred manner greater than 0.5 to 3 wt.%, is set,

Silicon Si: the optional addition of Si in higher contents impedes thediffusion of carbon, reduces the relative density and increases thestrength and elongation properties and toughness properties.Furthermore, an improvement in the cold-rollability could be seen byadding Si by alloying. Higher Si contents result in embrittlement of thematerial and negatively influence the hot- and cold-rollability and thecoatability e.g. by galvanising. Therefore, an Si content of 0 to 6 wt.%, preferably 0.05 to 3 wt. %, in a particularly preferred manner 0.1 to1.5 wt. %, is set.

Chromium Cr: the optional addition of Cr improves the strength andreduces the rate of corrosion, delays the formation of fen⁻Re andperlite and forms carbides. Higher contents result in impairment of theelongation properties. Therefore, a Cr content of 0 to 6 wt. %,preferably 0.1 to 4 wt. %, in a particularly preferred manner greaterthan 0.5 to 2.5 wt. %, is set.

Microalloy elements are generally added only in very small amounts. Incontrast to the alloy elements, they mainly act by precipitate formationbut can also influence the properties in the dissolved state. Smalladded amounts of the microalloy elements already considerably influencethe processing properties and final properties. Particularly in the caseof hot-forming, microalloy elements advantageously influence therecrystallisation behaviour and effect grain refinement,

Typical microalloy elements are vanadium, niobium and titanium. Theseelements can be dissolved in the iron lattice and form carbides,nitrides and carbonitrides with carbon and nitrogen.

Vanadium V and niobium Nb: these act in a grain-refining manner inparticular by forming carbides, whereby at the same time the strength,toughness and elongation properties are improved. Contents of more than1.5 wt. % or 1 wt. % do not provide any further advantages. For vanadiumand niobium, a minimum content of 0.005 wt. % and a maximum content of0.6 wt. % or 0.4 wt. % are optionally preferred, with a minimum contentof 0.01 wt. % and a maximum content of 0.3 wt. % or 0.1 wt. % beingparticularly preferred.

Titanium Ti: acts in a grain-refining manner as a carbide-forming agent,whereby at the same time the strength, toughness and elongationproperties are improved, and reduces the inter-crystalline corrosion.Contents of Ti of more than 1.5 wt. % impair the elongation properties,for which reason a maximum content of 1.5 wt. %, preferably 0.6 wt. %,in a particularly preferred manner 0.3 wt. %, is optionally set. Minimumcontents of 0.005 wt. %, preferably 0.01 wt. %, can be provided in orderto bind nitrogen and advantageously precipitate Ti.

Molybdenum Mo: acts as a carbide-forming agent, increases the strengthand increases the resistance to delayed crack formation and hydrogenembrittlement. High contents of Mo impair the elongation properties.Therefore, an Mo content of 0 to 3 wt. %, preferably 0.005 to 1.5 wt. %,in a particularly preferred manner greater than 0.01 to 0.6 wt. %, isoptionally set.

Tin Sn: tin increases the strength but, similar to copper, accumulatesbeneath the scale layer and at the grain boundaries at highertemperatures. This results, owing to the penetration into the grainboundaries, in the formation of lows melting phases and, associatedtherewith, in cracks in the microstructure and in solder brittleness,for which reason a maximum content of 0.5 wt. %, preferably less than0.2 wt. %, in a particularly preferred manner less than 0.05 wt. %, isoptionally provided.

Copper Cu: reduces the rate of corrosion and increases the strength.

Contents of above 3 wt. % impair the producibility by forming lowmelting phases during casting and hot roiling, for which reason amaximum content of 3 wt. %, preferably less than 0.5 wt. %, in aparticularly preferred manner less than 0.1 wt. %, is optionally set,

Tungsten W: acts as a carbide-forming agent and increases the strengthand heat resistance. Contents of W of more than 5 wt. % impair theelongation properties, for which reason a maximum content of 5 wt. % isoptionally set. A content of 0.01 wt. % to 3 wt. % is preferred, and 0.2to 1.5 wt. % is particularly preferred.

Cobalt Co: increases the strength of the steel, stabilises the austeniteand improves the heat resistance. Contents of more than 8 wt. % impairthe elongation properties. Therefore, the Co content is set to at most 8wt. %, preferably 0.01 to 5 wt. %, in a particularly preferred manner0.3 to 2 wt. %.

Zirconium Zr: acts as a carbide-forming agent and improves the strength.Contents of Zr of more than 0.5 wt. % impair the elongation properties.Therefore, a Zr content of 0 to 0.5 wt. %, preferably 0.005 to 0.3 wt.%, in a particularly preferred manner 0.01 to 0.2 wt. %, is set.

Tantalum Ta: tantalum acts in a similar manner to niobium as acarbide-forming agent in a grain-refining manner and thereby improvesthe strength, toughness and elongation properties at the same time.Contents of over 0.5 wt. % do not provide any further improvement in theproperties. Thus, a maximum content of 0.5 wt. % is optionally set.Preferably, a minimum content of 0.005 and a maximum content of 0.3 wt.% are set, in which the grain refinement can advantageously be produced.In order to improve economic feasibility and to optimise grainrefinement, a content of 0.01 wt. % to 0.1 wt. % is particularlypreferably sought.

Tellurium Te: tellurium improves the corrosion-resistance and themechanical properties and machinability. Furthermore, Te increases thesolidity of manganese sulphides (MnS) which, as a result, is lengthenedto a lesser extent in the rolling direction during hot rolling and coldrolling. Contents above 0.5 wt. % impair the elongation and toughnessproperties, for which reason a maximum content of 0.5 wt. % is set.Optionally, a minimum content of 0.005 wt. % and a maximum content of0.3 wt. % are set which advantageously improve the mechanical propertiesand increase the strength of MnS present. Furthermore, a minimum contentof 0.01 wt. % and a maximum content of 0.1 wt. % are preferred whichrender possible optimisation of the mechanical properties whilst at thesame time reducing alloy costs.

Boron B: boron delays the austenite conversion, improves the hot-formingproperties of steels and increases the strength at room temperature. Itachieves its effect even with very low alloy contents. Contents above0.15 wt. % greatly impair the elongation and toughness properties, forwhich reason the maximum content is set to 0.15 wt. %. Optionally, aminimum content of 0.001 wt. % and a maximum content of 0.08, preferablya minimum content of 0.002 wt. % and a maximum content of 0.01, is set,in order to advantageously use the strength-increasing effect of boron.

Phosphorus P: is a trace element, it originates predominately from ironore and is dissolved in the iron lattice as a substitution atom.Phosphorous increases the hardness by means of solid solution hardeningand improves the hardenability. However, attempts are generally made tolower the phosphorous content as much as possible because inter alia itexhibits a strong tendency towards segregation owing to its lowdiffusion rate and greatly reduces the level of toughness. Theattachment of phosphorous to the grain boundaries can cause cracks alongthe grain boundaries during hot rolling. Moreover, phosphorous increasesthe transition temperature from tough to brittle behaviour by up to 300°C. For the aforementioned reasons, the phosphorus content is limited tovalues of less than 0.1 wt. %, preferably less than 0.04 wt. %.

Sulphur S: like phosphorous, is bound as a trace element in the iron orebut in particular in the production route via the blast furnace processin the coke. It is generally not desirable in steel because it exhibitsa tendency towards extensive segregation and has a greatly embrittlingeffect, whereby the elongation and toughness properties are impaired. Anattempt is therefore made to achieve amounts of sulphur in the meltwhich are as low as possible (e.g. by deep desulphurisation). For theaforementioned reasons, the sulphur content is limited to values of lessthan 0.1 wt. %, preferably less than 0.02 wt. %.

Nitrogen N: N is likewise an associated element from steel production.In the dissolved state, it improves the strength and toughnessproperties in steels containing a higher content of manganese of greaterthan or equal to 4 wt. % Mn. Lower Mn-alloyed steels of less than 4 wt.% tend, in the presence of free nitrogen, to have a strong ageingeffect. The nitrogen diffuses even at low temperatures to dislocationsand blocks same. It thus produces an increase in strength associatedwith a rapid loss of toughness. Binding of the nitrogen in the form ofnitrides is possible e.g. by adding titanium or aluminium by alloying,wherein in particular aluminium nitrides have a negative effect upon thedeformation properties of the alloy in accordance with the invention.For the aforementioned reasons, the nitrogen content is limited to lessthan 0.1 wt. %, preferably less than 0.05 wt. %.

What is claimed is:
 1. A method for producing a component starting froma medium manganese flat steel product comprising 4 to less than 10 wt. %Mn, 0.0005 to 0.9 wt. % C, 0.02 to 10 wt. % Al, with the remainder beingiron including unavoidable steel-associated elements and with a TRIPeffect at room temperature, wherein the flat steel product is producedwith a microstructure which has an austenite content of 10 to 80%, 20 to90% martensite, ferrite and bainite, wherein at least 30% of themartensite is present as annealed martensite, said method comprising,the following steps to form the component: pre-heating the flat steelproduct without deformation to a temperature of 60° C. to 450° C.;deforming the pre-heated flat steel product by at least one deformingstep at a temperature of the flat steel product of 60° C. to 450° C.;and cooling the deformed flat steel product prior to at least one finaldeforming step at a temperature of 0° C. to −196° C.
 2. The method ofclaim 1, comprising an additional step of further deforming the cooledsteel flat product by one or a plurality of final deforming steps at atemperature of 0° C. to −196° C.
 3. The method of claim 1, furthercomprising pre-heating and/or cooling one side of the flat steelproduct.
 4. The method of claim 1, further comprising pre-heating and/orcooling both sides of the flat steel product.
 5. The method of claim 1,further comprising intermediately heating or intermediately cooling theflat steel product during deformation between deformation steps totemperatures, which are selected for the respective deforming steps,between −196° C. to below Ac3.
 6. The method of claim 1, wherein theflat steel product is deformed by roil deforming.
 7. The method of claim1, further comprising: forming the flat steel product into a tube; andlongitudinal seam welding or spiral seam welding the tube; and formingthe tube by internally high-pressure.
 8. The method of claim 7, furthercomprising annealing the formed tube.
 9. The method of claim 1, furthercomprising adding to the flat steel product by alloying, in wt. %: Si: 0to 6, Cr: 0 to 6, Nb: 0 to 1, V: 0 to 1.5, Ti: 0 to 1.5, Mo: 0 to 3, Sn:0 to 0.5, Cu: 0 to 3, W: 0 to 5, Co: 0 to 8, Zr: 0 to 0.5, Ta: 0 to 0.5,Te: 0 to 0.5, B: 0 to 0.15, P: less than 0,1, S: less than 0.1, N: lessthan 0.1.
 10. The method of claim 1, further comprising coatingmetallically, inorganically or organically the flat steel product or thecomponent.